After you throttle up just look inside the airplane and locate the black crispy thing and replace it with a 20amp one.

Actually, don't know what would happen physically, just be assured it won't work any more.

A speed 400 motor at full throttle will push the limit of a 10amp esc when it is under load so give it consideration as to the capacity of the esc.

20amp may be a little more than is needed for a speed 400 motor but you haven't wasted any money by being safe. I buy 30 amp escs so I can use them for bigger airplanes and bigger motors. I haven't bought any brushless motors yet, but when I do I will probably need to move up to at least a 50amp esc.

I'm not smart enough to live on the edge so I buy a little more than is needed, for a safety factor.